1. Field of the Invention
This invention is directed to a process for meta-xylene production using a molecular sieve catalyst system. More specifically, the invention is directed to a process for the catalytic conversion of ethylbenzene with minimized xylene isomerization, and subsequent purification to yield high-purity meta-xylene.
2. Description of the Prior Art
Meta-xylene is a valuable chemical intermediate, with a market price of about 45-60 cents/lb. The principal route for consumption involves oxidation of meta-xylene to isophthalic acid (or ester), the end use of which is for specialty polyester resins for high-performance plastics, such as those used in blow-molded products. For example, the isophthalic acid (ester) can be copolymerized with terephthalic acid (or its ester) and ethylene glycol to produce polyethyleneterephthalate (PET) resins. These resins find use in various products, including plastic bottles, and this market is expected to grow substantially in the coming years.
Meta-xylene may be derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C.sub.8 aromatics in such mixtures and their properties are:
Freezing Boiling Density Point (.degree. C.) Point (.degree. C.) (Kg/m.sup.3) Ethylbenzene -95.0 136.2 869.9 Para-xylene 13.2 138.5 863.9 Meta-xylene -47.4 138.8 866.3 Ortho-xylene -25.4 144.0 883.1
Calculated thermodynamic equilibria for the C.sub.8 aromatic isomers at 850.degree. F. (454.degree. C.) are:
 Component Proportion (wt %) Ethylbenzene 8.5 Para-xylene 22.5 Meta-xylene 48.0 Ortho-xylene 21.5 Total 100.0
Principal sources of the mixtures of C.sub.8 aromatics are catalytically reformed naphthas and pyrolysis distillates. The C.sub.8 aromatic fractions from these sources vary quite widely in composition but will usually be in the range of 10 wt % to 32 wt % ethylbenzene (EB) with the balance, xylenes, being divided approximately 50 wt % meta-xylene and 25 wt % each of para-xylene and orthxyene.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation, although this is a costly operation. Ortho-xylene may be separated by fractional distillation, and it is so produced commercially. Para-xylene may be separated from the mixed isomers by fractional crystallization, selective adsorption (e.g., the Parex process), or membrane separation.
As is evident in the table of properties above, the boiling point of ethylbenzene is very close to those of para-xylene and meta-xylene. Complete removal of ethylbenzene from the charge by conventional methods, e.g., distillation, is usually impractical. An ethylbenzene separation column may be used in the isomerizer-separator loop or the ethylbenzene may be converted catalytically in the isomerizer-separator loop.
In many processes for xylene isomerization, conversion of ethylbenzene is constrained by the need to hold conversion of xylenes to other compounds to acceptable levels. Thus, although catalytic removal of ethylbenzene is possible, operating conditions are still selected to balance the disadvantages of xylene loss by transalkylation with the conversion of ethylbenzene.
There is currently one commercially practiced method for production of meta-xylene, i.e., the Mitsubishi Gas Chemical Company (MGCC) process. See Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed., Vol. 24, p. 727 (1984), and the publications cited therein, This separations process requires treating a mixture of isomerized xylenes with HF-BF.sub.3. Two layers are formed, i.e., a 1:1 molecular complex, meta-xylene-HBF.sub.3 layer, and an organic layer containing the remaining xylenes. Meta-xylene is then recovered in 99% purity by thermal decomposition of the meta-xylene-HBF.sub.3 complex. Although this method is used commercially by MGCC, the difficulties in dealing with HF-BF.sub.3 make this process costly.
UOP has recently announced a sorption-based meta-xylene separations process designated MX-SORBEX but its performance is not presently known.
Thus, the state of the art of meta-xylene production is such that demand for this material continues to increase, while methods for preparing the material lag in commercial practicability. Presently, the processes for producing meta-xylene are limited by cost and purity considerations.
It is, therefore, highly desirable to provide a commercially acceptable process for the production of meta-xylene in substantially pure form. It would also be desirable to provide a process for deriving high-purity meta-xylene from products of other types of reactions to take advantage of readily available resources.
In view of the above considerations, it is clear that existing catalysts and processes for shape selective hydrocarbon conversion are critical to improving the quality and yield of materials suitable for commercial manufacturing. Accordingly, it is one of the purposes of this invention to overcome the above limitations in shape selective hydrocarbon conversion processing, by providing a process for shape selective hydrocarbon conversion processes to produce high-purity meta-xylene.